Properly sanitize Windows reserved names in evaluator paths

main/eval/src/mill/eval/EvaluatorPaths.scala

@@ -28,7 +28,7 @@ object EvaluatorPaths {
   ): EvaluatorPaths = {
     val refinedSegments = foreignSegments.map(_ ++ segments).getOrElse(segments)
     val segmentStrings = makeSegmentStrings(refinedSegments)
-    val targetPath = workspacePath / segmentStrings
+    val targetPath = workspacePath / segmentStrings.map(sanitizePathSegment)
     EvaluatorPaths(
       targetPath / os.up / s"${targetPath.last}.dest",
       targetPath / os.up / s"${targetPath.last}.json",
@@ -39,4 +39,14 @@ object EvaluatorPaths {
       workspacePath: os.Path,
       task: NamedTask[_]
   ): EvaluatorPaths = resolveDestPaths(workspacePath, task.ctx.segments, task.ctx.foreign)
+
+  // case-insensitive match on reserved names
+  private val ReservedWinNames =
+    raw"^([cC][oO][nN]|[pP][rR][nN]|[aA][uU][xX]|[nN][uU][lL]|[cC][oO][mM][0-9¹²³]|[lL][pP][tT][0-9¹²³])($$|[.].*$$)".r
+  def sanitizePathSegment(segment: String): os.PathChunk = {
+    segment match {
+      case ReservedWinNames(keyword, rest) => s"${keyword}~${rest}"
+      case s => s
+    }
+  }
 }

main/eval/test/src/mill/eval/EvaluatorPathsTests.scala

@@ -0,0 +1,26 @@
+package mill.eval
+
+import utest._
+
+object EvaluatorPathsTests extends TestSuite {
+
+  override def tests: Tests = Tests {
+    "sanitizedPathSegment" - {
+      "WindowsReservedNames" - {
+        val replace = Seq(
+          "com1.json" -> "com1~.json",
+          "LPT¹" -> "LPT¹~"
+        )
+        val noReplace = Seq(
+          "con10.json"
+        )
+        for {
+          (segment, result) <- replace ++ noReplace.map(s => (s, s))
+        } yield {
+          EvaluatorPaths.sanitizePathSegment(segment).toString ==> result
+          (segment, result)
+        }
+      }
+    }
+  }
+}